Power transmission apparatus of a hybrid electric vehicle

This application claims priority to and the benefit of Korean Patent Application No. 10-2016-0137790 filed in the Korean Intellectual Property Office on Oct. 21, 2016, the entire contents of which are incorporated herein by reference.

(a) Field

The present disclosure relates to a power transmission apparatus for a hybrid electric vehicle.

(b) Description of the Related Art

Environmentally-friendly vehicle technology is an example of the core technology that controls the future of the automobile industry, and advanced car makers have focused their energy on the development of an environmentally-friendly vehicle to meet environmental and fuel efficiency regulations.

In particular, car makers have developed an electric vehicle (“EV”), a hybrid electric vehicle (“HEV”), a fuel cell electric vehicle (“FCEV”), and the like, as future vehicle technologies.

Because such vehicles have various technological restrictions such as a weight and cost, the vehicle makers have turned to the hybrid electric vehicle as a way to meet exhaust gas regulations and improve fuel efficiency performance, and have entered into keen competition for commercializing the hybrid electric vehicle.

The hybrid electric vehicle is a vehicle using two or more power sources. The power sources may be combined by various methods. For instance, the power sources may include a motor/generator driven by electric energy and may also include a fossil fuel engine, such as either a gasoline engine or a diesel engine.

The hybrid electric vehicle generally uses, as a primary power source, both (i) a motor/generator having a relatively excellent torque at low speeds and (ii) an engine having a relatively excellent torque at high speeds.

At low speeds, the hybrid electric vehicle's operation of the fossil fuel engine stops, and the motor/generator is used instead. As a result, the hybrid electric vehicle can provide high fuel efficiency and a reduction of exhaust gas.

A double clutch transmission (“DCT”) is an example of a transmission useful in a hybrid electric vehicle. A DCT includes two clutches applied to a manual transmission scheme, and thereby enhances efficiency and convenience.

The DCT uses two separate clutches for odd-numbered shift-stages and even-numbered shift-stages, and thereby improves continuity in torque transmission by alternatingly operating these two clutches.

However, in some scenarios when starting a vehicle, such as starting a vehicle on a slanted surface (which can cause a rearward slip of the vehicle), use of the DCT results in a relatively high degree of clutch wear and energy loss. In addition, the DCT typically is controlled with a short shift-control period due to low heat capacity, which can easily cause shift shock to occur.

In addition, in order to use a DCT in a hybrid electric vehicle, an appropriate arrangement of a motor/generator as a power source must be devised.

The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.

The present disclosure provide, for a hybrid electric vehicle, a power transmission apparatus having the following advantages: smooth starting and shifting of the vehicle, improved fuel consumption, and improved acceleration performance.

An example embodiment includes a power transmission apparatus of a hybrid electric vehicle having an engine and a motor/generator as power sources. The power transmission apparatus may include an input unit, a torque converting unit, and a shifting output unit. The input unit may include a first input shaft selectively connected with the engine and the motor/generator and fixedly disposed with one input gear; a second input shaft selectively connected with the motor/generator; and a third input shaft fixedly disposed with at least one input gear. The torque converting unit may include a planetary gear set having a first rotational element connected with the second input shaft, a second rotational element connected with the third input shaft, and a third rotational element connected with the first input shaft. The shifting output unit may shift torques received from the first and third input shafts and output a shifted torque.

The second and third input shafts of the input unit may be formed as a hollow shaft, and the first, second, and third input shafts may be coaxially disposed in a radial direction in an order of the first, second, and third input shafts.

The torque converting unit may be disposed at a downstream portion of the first, second, and third input shafts.

The torque converting unit may be a single pinion planetary gear set having a sun gear as the first rotational element, a planet carrier as the second rotational element, and a ring gear as the third rotational element.

The shifting output unit may include first and second shifting output devices. The first shifting output device may include a first output shaft disposed in parallel with the first input shaft, two shifting gears rotatably disposed on the first output shaft and externally engaged with two input gear fixedly formed on the third input shaft, and a first output gear fixedly disposed at an upstream portion of the first output shaft. The second shifting output device may include a second output shaft disposed in parallel with the first input shaft, an idle shaft rotatably disposed on the second output shaft, one shifting gear fixedly disposed on the idle shaft, selectively connected with the second output shaft, and externally engaged with the one input gear fixedly disposed on the first input shaft, and a second output gear fixedly disposed at an upstream portion of the second output shaft.

The at least one input gear fixedly disposed on the third input shaft may include a first input gear acting as an input gear for the forward first speed and a second input gear acting as an input gear for the forward third speed. The one input gear fixedly disposed on the first input shaft may be a third input gear acting as an input gear for the forward second speed.

A power transmission apparatus of a hybrid electric vehicle according to an example embodiment of the present disclosure shows an effective improvement in fuel consumption by providing for three shift-stages in a fixed gear ratio mode by using the engine as a primary power source and the motor/generator as an auxiliary power source, two shift-stages in a variable gear ratio mode enabling electronic continuously variable shifting by using the engine and the motor/generator as power sources, and three shift-stages by only using the motor/generator as a power source.

In particular, driving efficiency may be enhanced by operating a power transmission apparatus in the variable gear ratio mode for city driving, and in the fixed gear ratio mode for high speed driving.

In addition, smooth starting of a vehicle may be achieved by having the engine and the motor/generator connected with respective rotational elements of the planetary gear set.

When the motor/generator fails, a vehicle still may be driven by the torque of the engine.

In the fixed gear ratio mode, the torque of the motor/generator may be used as an auxiliary power source, thereby improving acceleration performance.

The reverse driving speed may be achieved by reversing the motor/generator.

Further, effects that can be obtained or expected from example embodiments of the present disclosure are directly or suggestively described in the following detailed description. That is, various effects expected from example embodiments of the present disclosure will be described in the following detailed description.

FIG. 1 is a schematic diagram of a power transmission apparatus of a hybrid electric vehicle according to a first example embodiment.

FIG. 2 is an operational chart for respective shift-stages of a power transmission apparatus of a hybrid electric vehicle according to a first example embodiment.

FIG. 3 is a schematic diagram of a power transmission apparatus of a hybrid electric vehicle according to a second example embodiment.

The present disclosure will be described more fully hereinafter with reference to the accompanying drawings, in which example embodiments of the disclosure are shown.

The drawings and description are to be regarded as illustrative in nature and not restrictive, and like reference numerals designate like elements throughout the specification.

In this detailed description, a “front” or “upstream” of the transmission is referred to as a side proximal to the engine, and a “rear” or “downstream” of the transmission is referred to a side distal to the engine.

In the following description, names of components may be divided into first, second, and the like to distinguish parts having the same name but which need not necessarily occur in a particular order.

1. First Example Power Transmission Apparatus

FIG. 1 is a schematic diagram of a power transmission apparatus of a hybrid electric vehicle according to an example embodiment.

Referring to FIG. 1 , a power transmission apparatus of a hybrid electric vehicle utilizes an engine ENG and a motor/generator MG as power sources, and includes an input unit, a torque converting unit, and a shifting output unit.

The engine ENG is a primary power source, and may be a gasoline engine or a diesel engine.

The motor/generator MG is an electric supplementary drive unit (“ESDU”), and functions as a motor that generates a driving torque and also as a generator that generates electric energy producing a mechanical reactive force. The motor/generator MG includes a stator ST fixed to a transmission housing H and a rotor RT rotatable inside the stator, where the rotor RT is directly connected with a motor output shaft MOS.

The input unit includes first, second, and third input shafts IS 1 , IS 2 , and IS 3 .

The first input shaft IS 1 is formed as a solid shaft and disposed on the same axis as an engine output shaft EOS. An upstream portion of the first input shaft IS 1 is selectively connected with the engine output shaft EOS and the motor output shaft MOS respectively.

The second input shaft IS 2 is formed as a hollow shaft, and rotatably and coaxially disposed at an external circumference of the first input shaft IS 1 . An upstream portion of the second input shaft IS 2 is selectively connected with the motor output shaft MOS.

The third input shaft IS 3 is formed as a hollow shaft, and rotatably and coaxially disposed at an external circumference of the second input shaft IS 2 .

First and second input gears G 1 and G 2 are fixedly disposed on the third input shaft IS 3 , and a third input gear G 3 is fixedly disposed on the first input shaft IS 1 .

The first, second, and third input gears G 1 , G 2 , and G 3 act as input gears for respective shift-stages, where the first input gear G 1 acts as an input gear for realizing the forward first speed, the second input gear G 2 as an input gear for realizing the forward third speed, and the third input gear G 3 as an input gear for realizing the forward second speed.

A clutch unit including an engine clutch ECL and first and second clutches CL 1 and CL 2 is disposed between first and second input shafts IS 1 and IS 2 and the power sources of the engine ENG and the motor/generator MG.

The engine clutch ECL is disposed between the engine output shaft EOS and the first input shaft IS 1 , and selectively transmits torque from the engine ENG to the first input shaft IS 1 .

The first clutch CL 1 is disposed between the motor output shaft MOS and the first input shaft IS 1 , and selectively transmits torque from the motor/generator MG to the first input shaft IS 1 .

The second clutch CL 2 is disposed between the motor output shaft MOS and the second input shaft IS 2 , and selectively transmits torque from the motor/generator MG to the second input shaft IS 2 .

The engine clutch ECL and the first and second clutches CL 1 and CL 2 may comprise multi-plate hydraulic pressure friction devices that are frictionally engaged by hydraulic pressure.

The torque converting unit is formed as a planetary gear set PG that is a single pinion planetary gear set having three rotational elements.

The three rotational elements of the planetary gear set PG include first, second, and third rotational elements N 1 , N 2 , and N 3 , where the first rotational element N 1 is a sun gear, the second rotational element N 2 is a planet carrier PC that rotatably supports one or more pinion gears externally engaged with the sun gear S, and the third rotational element N 3 is a ring gear R engaged with the one or more pinion gears.

The first rotational element N 1 is directly connected with the second input shaft IS 2 , the second rotational element N 2 is directly connected with the third input shaft IS 3 , and the third rotational element N 3 is directly connected with the first input shaft IS 1 .

When torques of the engine ENG and the motor/generator MG are selectively transmitted to the first rotational element N 1 and the third rotational element N 3 through the first and second input shafts IS 1 and IS 2 , such input torques are converted and output to the third input shaft IS 3 through the second rotational element N 2 .

The shifting output unit includes first and second shifting output devices OUT 1 and OUT 2 that receive torques from the first, second, and third input gears G 1 , G 2 , and G 3 on the third and first input shafts IS 3 and IS 1 and output shifted torques.

The first shifting output device OUT 1 includes a first output shaft OS 1 and forward first speed and third speed shifting gears D 1 and D 3 . The first output shaft OS 1 is disposed in parallel with the first input shaft IS 1 , and the forward first speed and third speed shifting gears D 1 and D 3 are externally engaged with the first and second input gears G 1 and G 2 respectively.

The forward first speed and third speed shifting gears D 1 and D 3 are rotatably disposed on the first output shaft OS 1 .

The first shifting output device OUT 1 further includes a synchronizing unit, a first synchronizer SL 1 in this embodiment, that is disposed on the first output shaft OS 1 and selectively synchronizes the forward first speed and third speed shifting gears D 1 and D 3 to the first output shaft OS 1 .

The forward first speed shifting gear D 1 is externally engaged with the first input gear G 1 , and the forward third speed shifting gear D 3 is externally engaged with the second input gear G 2 .

The torque shifted at the first shifting output device OUT 1 is transmitted to a differential device DIFF through a first output gear OG 1 fixed to an upstream portion of the first output shaft OS 1 .

The first synchronizer SL 1 may be formed according to known methods, and the sleeve SLE 1 applied to the first synchronizer SL 1 may be operated by an actuator (not shown) that may be controlled by a transmission control unit.

The second shifting output device OUT 2 includes a second output shaft OS 2 , an idle shaft IDS, and a forward second speed shifting gear D 2 . The second output shaft OS 2 is disposed in parallel with the first input shaft IS 1 , the idle shaft IDS is rotatably disposed on the second output shaft OS 2 , and the forward second speed shifting gear D 2 is disposed on the idle shaft IDS and externally engaged with the third input gear G 3 on the first input shaft IS 1 .

The forward second speed shifting gear D 2 is fixedly disposed on the idle shaft IDS.

The second shifting output device OUT 2 further includes a third clutch CL 3 that is disposed between the second output shaft OS 2 and the idle shaft IDS and selectively connects the forward second speed shifting gear D 2 to the second output shaft OS 2 .

The torque shifted at the second shifting output device OUT 2 is transmitted to a differential device DIFF through a second output gear OG 2 fixed to an upstream portion of the second output shaft OS 2 .

The third clutch CL 3 disposed between the second output shaft OS 2 and the idle shaft IDS selectively transmits a torque from the idle shaft IDS to the second output shaft OS 2 .

The third clutch CL 3 may be a multi-plate hydraulic pressure friction device that is frictionally engaged by hydraulic pressure.

A power transmission apparatus of a hybrid electric vehicle according to this example embodiment may enable various modes, for example, a fixed gear ratio mode that simulates a double clutch transmission, a variable gear ratio mode (“eCVT mode”), and electric vehicle mode (“EV mode”).

2. Shift-Stages of the First Example Power Transmission Apparatus

FIG. 2 is an operational chart for respective shift-stages of a power transmission apparatus of a hybrid electric vehicle according to a first example embodiment. Shifting operation of the power transmission apparatus is hereinafter described in detail with reference to FIG. 2 .

a. Engine Starting Mode

In an engine starting mode, while the forward first speed shifting gear D 1 is synchronized to the first output shaft OS 1 by operating the first sleeve SLE 1 of the first synchronizer SL 1 , the engine clutch ECL and the first clutch CL 1 are operated.

Next, a driving torque of the motor/generator MG is transmitted to the engine ENG through the motor output shaft MOS, the first clutch CL 1 , the first input shaft IS 1 , the engine clutch ECL, and the engine output shaft EOS, and the engine ENG is started.

b. Neutral Mode

In a neutral mode, while the forward first speed shifting gear D 1 is synchronized to the first output shaft OS 1 by operating the first sleeve SLE 1 of the first synchronizer SL 1 , the engine clutch ECL is operated.

In this case, although the torque of the engine ENG is supplied to the third rotational element N 3 of the planetary gear set PG through the first input shaft IS 1 , the first rotational element N 1 is freely rotatable. Thus, the second rotational element N 2 does not output any driving torque and therefore a neutral position is enabled.

c. Neutral Position Battery Charging Mode

A neutral position battery charging mode is enabled while the engine ENG is started and running. While the forward first speed shifting gear D 1 is synchronized to the first output shaft OS 1 by operating the first sleeve SLE 1 of the first synchronizer SL 1 , the engine clutch ECL and the second clutch CL 2 are operated.

Next, the torque of the engine ENG is input to the third rotational element N 3 of the planetary gear set PG through the first input shaft IS 1 , and the second rotational element N 2 acts as a fixed element by being connected to the first output shaft OS 1 due to the operation of the first synchronizer SL 1 .

In this case, the first rotational element N 1 reversely rotates at a high speed, and drives the motor/generator MG through second clutch CL 2 to generate electricity and thereby enable electric charging.

d. Fixed Gear Ratio Mode 1

In a fixed gear ratio mode, the engine ENG supplies a primary drive-torque and the motor/generator MG supplies an auxiliary drive-torque.

In a fixed gear ratio mode 1 , the forward first speed shifting gear D 1 is synchronized to the first output shaft OS 1 by operating the first sleeve SLE 1 of the first synchronizer SL 1 , and the engine clutch ECL and the first and second clutches CL 1 and CL 2 are operated.

This allows the torque of the engine ENG and a part of the torque of the motor/generator MG to be input to the third rotational element N 3 of the planetary gear set PG, and a part of the torque of the motor/generator MG to be input to the first rotational element N 1 of the planetary gear set PG through the second input shaft IS 2 .

Consequently, torques are simultaneously input to the first and third rotational elements N 1 and N 3 of the planetary gear set PG, and the planetary gear set PG integrally rotates, thereby outputting a same input torque to the third input shaft IS 3 through the second rotational element N 2 .

Next, a shifted torque is transmitted to the first output shaft OS 1 through the forward first speed shifting gear D 1 externally engaged with the first input gear G 1 on the third input shaft IS 3 , and transmitted to the differential DIFF through the first output gear OG 1 on the first output shaft OS 1 .

e. Fixed Gear Ratio Mode 2

To transition to fixed gear ratio mode 2 from the fixed gear ratio mode 1 , the synchronous connection of the forward first speed shifting gear D 1 to the first output shaft OS 1 is released by releasing the first sleeve SLE 1 of the first synchronizer SL 1 , the second clutch CL 2 is released, and the third clutch CL 3 is operated.

This allows the torque of the engine ENG and a part of the torque of the motor/generator MG to be input to the third rotational element N 3 of the planetary gear set PG through the first input shaft IS 1 .

Next, the torque is transmitted to the idle shaft IDS and the second output shaft OS 2 through the forward second speed shifting gear D 2 externally engaged with the third input gear G 3 on the first input shaft IS 1 , and is transmitted to the differential DIFF through the second output gear OG 2 of the second output shaft OS 2 .

In this case, the planetary gear set PG does not affect shifting of the power transmission apparatus because the first and second rotational elements N 1 and N 2 may freely rotate.

f. Fixed Gear Ratio Mode 3

To transition to fixed gear ratio mode 3 from fixed gear ratio mode 2 , the forward third speed shifting gear D 3 is synchronized to the first output shaft OS 1 by operating the first sleeve SLE 1 of the first synchronizer SL 1 , the third clutch CL 3 is released, and the second clutch CL 2 is operated.

This allows the torque of the engine ENG and a part of the torque of the motor/generator MG to be input to the third rotational element N 3 of the planetary gear set PG, and a part of the torque of the motor/generator MG to bes input to the first rotational element N 1 of the planetary gear set PG through the second input shaft IS 2 .

Consequently, torques are simultaneously input to the first and third rotational elements N 1 and N 3 of the planetary gear set PG, and the planetary gear set PG integrally rotates, thereby outputting a same input torque to the third input shaft IS 3 through the second rotational element N 2 .

Next, a shifted torque is transmitted to the first output shaft OS 1 through the forward third speed shifting gear D 3 externally engaged with the second input gear G 2 on the third input shaft IS 3 , and transmitted to the differential DIFF through the first output gear OG 1 on the first output shaft OS 1 .

g. Variable Gear Ratio Mode 1

In a variable gear ratio mode, both torque of the motor/generator MG and the torque of the engine ENG are used as a driving power source.

Motor/generator MG may rotate at a different speed from the engine ENG, and in this case, the different speeds of the motor/generator MG and the engine ENG may be input to the planetary gear set PG such that the planetary gear may output a variable gear ratio thereby achieving an electronic continuously variable transmission (“eCVT”) control.

That is, in a variable gear ratio mode 1 , the forward first speed shifting gear D 1 is synchronized to the first output shaft OS 1 by operating the first sleeve SLE 1 of the first synchronizer SL 1 , and the engine clutch ECL and the second clutch CL 2 are operated.

In this case, by the operation of the engine clutch ECL, the torque of the engine ENG is input to the third rotational element N 3 through the first input shaft IS 1 of the planetary gear set PG, and by the operation of the second clutch CL 2 , the torque of the motor/generator MG is input to the first rotational element N 1 through the second input shaft IS 2 .

Consequently, torques input to the first rotational element N 1 and the third rotational element N 3 react at the planetary gear set PG, and a shifted torque is output to the third input shaft IS 3 through the second rotational element N 2 .

Next, the shifted torque is transmitted to the first output shaft OS 1 through the forward first speed shifting gear D 1 externally engaged with the first input gear G 1 on the third input shaft IS 3 , and transmitted to the differential DIFF through the first output gear OG 1 on the first output shaft OS 1 .

h. Variable Gear Ratio Mode 2

To transition to variable gear ratio mode 2 from variable gear ratio mode 1 , the synchronous connection of the forward first speed shifting gear D 1 to the first output shaft OS 1 is released by operating the first sleeve SLE 1 of the first synchronizer SL 1 , and the first output shaft OS 1 and the forward third speed shifting gear D 3 are connected.

Shifting from variable gear ratio mode 1 to variable gear ratio mode 2 is not directly performed, but instead achieved by intermediately operating the clutches CL 1 , CL 2 , and CL 3 in a state of the fixed gear ratio mode 2 , so as to prevent shift shock.

That is, the first, second, and third clutches CL 1 , CL 2 , and CL 3 are operated from the state of the variable gear ratio mode 1 to the state of the fixed gear ratio mode 2 . Subsequently, the synchronous connection of the first output shaft OS 1 and the forward first speed shifting gear D 1 is released by releasing the first synchronizer SL 1 , and the first output shaft OS 1 and the forward third speed shifting gear D 3 are synchronized by operating the first synchronizer SL 1 .

When the first output shaft OS 1 and the forward third speed shifting gear D 3 are synchronized, the operation of the first clutch CL 1 and the third clutch CL 3 are released, and the second clutch CL 2 is operated.

In this case, the torque of the engine ENG is input to the third rotational element N 3 of the planetary gear set PG through the first input shaft IS 1 by the operation of the engine clutch ECL, and the torque of the motor/generator MG is input to the first rotational element N 1 through the second input shaft IS 2 by the operation of the second clutch CL 2 .

Consequently, torques input to the first rotational element N 1 and the third rotational element N 3 react at the planetary gear set PG, and a shifted torque is output to the third input shaft IS 3 through the second rotational element N 2 .

Next, a shifted torque is transmitted to the first output shaft OS 1 through the forward third speed shifting gear D 3 externally engaged with the second input gear G 2 on the third input shaft IS 3 , and transmitted to the differential DIFF through the first output gear OG 1 on the first output shaft OS 1 .

i. EV Mode 1

In an electric vehicle mode (“EV mode”), the engine ENG is stopped, and only the torque of the motor/generator MG is used to drive a vehicle.

In an EV mode 1 , the forward first speed shifting gear D 1 is synchronized to the first output shaft OS 1 by operating the first sleeve SLE 1 of the first synchronizer SL 1 , and the first and second clutches CL 1 and CL 2 are operated.

This allows the torque of the motor/generator MG to be simultaneously input to the third rotational element N 3 and the first rotational element N 1 through the first input shaft IS 1 and the second input shaft IS 2 .

Consequently, the torque of the motor/generator MG is simultaneously input to the first and third rotational elements N 1 and N 3 of the planetary gear set PG, and the planetary gear set PG integrally rotates, thereby outputting a same torque of the motor/generator MG to the third input shaft IS 3 through the second rotational element N 2 .

Next, a shifted torque is transmitted to the first output shaft OS 1 through the forward first speed shifting gear D 1 externally engaged with the first input gear G 1 on the third input shaft IS 3 , and transmitted to the differential DIFF through the first output gear OG 1 on the first output shaft OS 1 .

In such an EV mode 1 , a gear ratio of the first input gear G 1 and the forward first speed shifting gear D 1 is activated.

j. EV Mode 2

To transition to EV mode 2 from EV mode 1 , the second clutch CL 2 is released, the synchronous connection of the forward first speed shifting gear D 1 to the first output shaft OS 1 is released by releasing the first sleeve SLE 1 of the first synchronizer SL 1 , and the third clutch CL 3 is operated.

This allows the torque of the motor/generator MG to be input only to the first input shaft IS 1 .

Next, a shifted torque is transmitted to the second output shaft OS 2 connected with the idle shaft IDS by the third clutch CL 3 through the forward second speed shifting gear D 2 externally engaged with the third input gear G 3 on the first input shaft IS 1 , and is transmitted to the differential DIFF through the second output gear OG 2 of the second output shaft OS 2 .

In this case, the planetary gear set PG does not affect to a shifting of the power transmission apparatus since the first and second rotational elements N 1 and N 2 may freely rotate.

k. EV Mode 3

To transition to EV mode 3 from the EV mode 2 , the third clutch CL 3 is released, the forward third speed shifting gear D 3 is synchronized to the first output shaft OS 1 by operating the first sleeve SLE 1 of the first synchronizer SL 1 , and the second clutch CL 2 is operated.

This allows the torque of the motor/generator MG to be simultaneously input to the third rotational element N 3 and the first rotational element N 1 through the first input shaft IS 1 and the second input shaft IS 2 .

Consequently, the torque of the motor/generator MG is simultaneously input to the first and third rotational elements N 1 and N 3 of the planetary gear set PG, and the planetary gear set PG integrally rotates, thereby outputting a same torque of the motor/generator MG to the third input shaft IS 3 through the second rotational element N 2 .

Next, a shifted torque is transmitted to the first output shaft OS 1 through the forward third speed shifting gear D 3 externally engaged with the second input gear G 2 on the third input shaft IS 3 , and transmitted to the differential DIFF through the first output gear OG 1 on the first output shaft OS 1 .

l. EV Mode R (Reverse Speed; Motor is Reversely Driven)

In an EV mode R, only a reverse torque of the motor/generator MG is used to drive a vehicle while the engine ENG is stopped.

In the EV mode R, while the engine ENG is stopped, the forward first speed shifting gear D 1 is synchronized to the first output shaft OS 1 by operating the first sleeve SLE 1 of the first synchronizer SL 1 , and the first and second clutches CL 1 and CL 2 are operated.

This allows a reverse torque of the motor/generator MG to be simultaneously input to the third rotational element N 3 and the first rotational element N 1 through the first input shaft IS 1 and the second input shaft IS 2 .

Consequently, the torque of the motor/generator MG is simultaneously input to the first and third rotational elements N 1 and N 3 of the planetary gear set PG, and the planetary gear set PG integrally rotates reversely, thereby outputting a same torque of the motor/generator MG to the third input shaft IS 3 through the second rotational element N 2 .

Next, a shifted torque is transmitted to the first output shaft OS 1 through the forward first speed shifting gear D 1 externally engaged with the first input gear G 1 on the third input shaft IS 3 , and transmitted to the differential DIFF through the first output gear OG 1 on the first output shaft OS 1 .

Shifting operation has been described above with reference to an example of sequential upshifting, and it will be understood that shifting operation of sequential downshifting may be achieved by an opposite control.

In addition, the reverse speed has not been described in connection with the fixed gear ratio mode and the variable gear ratio mode, because a separate reverse speed shifting device is not included.

Thus, the reverse speed may be achieved only in the EV mode of a power transmission apparatus of a hybrid electric vehicle according to a first example embodiment.

3. Second Example Power Transmission Apparatus

FIG. 3 is a schematic diagram of a power transmission apparatus of a hybrid electric vehicle according to a second example embodiment.

Referring to FIG. 3 , the second example power transmission apparatus of a hybrid electric vehicle differs from the first example power transmission apparatus in that the third input gear G 3 and the forward second speed shifting gear D 2 externally engaged thereto are disposed at different locations.

In more detail, the third input gear G 3 and the forward second speed shifting gear D 2 are disposed rearward of the planetary gear set PG in the second example power transmission apparatus, whereas the third input gear G 3 and the forward second speed shifting gear D 2 are disposed forward of the planetary gear set PG in the first example power transmission apparatus.

Thus, the first and second example power transmission apparatuses differ only in the locations of the third input gear G 3 and the forward second speed shifting gear D 2 . Other arrangements and shifting operations are the same for both the first and second example power transmission apparatuses.

As described above, a power transmission apparatus for a hybrid electric vehicle shows an effective improvement of fuel consumption by providing three shift-stages in a fixed gear ratio mode by using the engine ENG as a primary power source and the motor/generator MG as an auxiliary power source, two shift-stages in a variable gear ratio mode enabling electronic continuously variable shifting by using the engine ENG and the motor/generator MG as power sources, and three shift-stages by only using the motor/generator as a power source.

In particular, driving efficiency may be enhanced by operating a power transmission apparatus in the variable gear ratio mode for city driving, and in the fixed gear ratio mode for high speed driving.

In addition, smooth starting of a vehicle may be achieved by the engine ENG and the motor/generator connected with respective rotational elements of the planetary gear set.

When the motor/generator fails, a vehicle may be driven by the torque of the engine ENG.

In the fixed gear ratio mode, the torque of the motor/generator may be used as an auxiliary power source, thereby improving an acceleration performance.

The reverse speed driving may be achieved by reversely driving the motor/generator MG.

While this disclosure has been described in connection with practical example embodiments, it is to be understood that the disclosure is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

5. Description of Symbols

ENG: engine

CL 1 , CL 2 , CL 3 : first, second, and third clutches

ECL: engine clutch

D 1 , D 2 , D 3 : first, second, and third speed shifting gears

EOS: engine output shaft

G 1 , G 2 , G 3 : first, second, and third input gears

MG: motor/generator

MOS: motor output shaft

IS 1 , IS 2 , IS 3 : first, second, and third input shafts

OS 1 , OS 2 : first and second output shafts

OUT 1 , OUT 2 : first and second shifting output devices

PG: planetary gear set

SL 1 : first synchronizer